There remains a great need to efficiently generate programmable voltage pulses for delivery to a patient location from an implanted device, e.g., to deliver pacing pulses from an implanted pacemaker to a patient's heart. The generation of stimulus pulses has remained the subject of continuing research and development, largely because the efficiency of generation is a primary determinant of energy expenditure, and thus of device longevity. Battery development has been ongoing, but so too have been the demands on the implanted device for extra features such as telemetry capability, processing and storage of diagnostic data, performing various logic functions, etc. With the increasing demands for performance capabilities, and of course longevity, improving the efficiency of delivering the periodic stimulus pulses remains a major goal, for implantable medical devices in particular.
The conventional prior art circuit used in an implantable device such as a pacemaker, defibrillator, nerve stimulator, etc., utilizes one or more large storage capacitors which are discharged to the load through a coupling capacitor to provide the stimulus pulse, the storage capacitor or capacitors then being recharged between pulses. In a pacemaker, such storage capacitors are typically in the range of 10-22 F; such a large capacitor occupies an undesirably large volume, is expensive, and is relatively inefficient to recharge. In older circuits, in order to provide a programmable voltage output, a single large storage capacitor was pumped to a programmable value with small capacitors. However, such an arrangement had the disadvantage that the voltage value could not be changed quickly. Subsequently, a next system was developed where several large capacitors, each charged to the battery voltage or half battery voltage, were stacked during the pulse duration so as to provide the programmed voltage output value. However, in this situation, the effective capacitance during pulse delivery, and thus the output impedance, is a variable, depending on the programmed output voltage value. Further, stacking of capacitors results in a smaller effective capacitance, such that there is increased voltage reduction while current flows to the load, making it more difficult to maintain a long pulse width without a sagging pulse voltage level. Further, and importantly, efficiency is compromised, due to the need to recharge significantly.
There thus remains a significant need in the implantable device art for an improved output generator which can provide more efficient delivery of energy to the target location, and also meet the other demands of a modern device. For example, such an improved energy efficient output generator must also be programmable to deliver stimulus pulses at different output levels. It is a further object to provide, in a battery-driven device, an efficient voltage pulse generator, where the voltage level of the pulse is maintained substantially constant, e.g., within 10% for the duration of the pulse.